Determination of the H-2/H-1, O-17/O-16, and O-18/O-16 isotope ratios in water by means of tunable diode laser spectroscopy at 1.39 mu m

We demonstrate the feasibility of the accurate and simultaneous measurement of the H-2/H-1, O-17/O-16, and O-18/O-16 isotope ratios in water vapor by means of tunable diode laser spectroscopy. The absorptions are due to the nu(1) + nu(3) combination band, observed using a room temperature, distributed feedback (DFB) diode laser at 1.39 mum. The precision of the instrument is approximately 3, 1, and 0.5% for the H-2, O-17, and O-18 isotope ratios, respectively, and is at present limited by residual optical feedback to the laser. The signal-to-noise, however, is superior to that obtained in a similar experiment using a color center laser at 2.7 mum. Replacing the current laser with a better unit, we are confident that a precision well below 1parts per thousand is attainable for all three isotope ratios. The diode laser apparatus is ideally suited for applications demanding a reliable, cheap, and/or portable instrument, such as the biomedical doubly labeled water method and atmospheric sensing. (C) 2002 Elsevier Science B.V. All rights reserved.